Abstract
The susceptibility of four populations of Varroa destructor to the organophosphorous
acaricide coumaphos was studied. The mites were taken from apiaries kept in Lombardy
(northern Italy) and tested with a laboratory assay on paraffin wax with known
concentrations of coumaphos. The susceptibility of two populations to coumaphos
was close to that of susceptible populations, indicated in the literature
(LC50 at 24 hours: 12.6 g/g); a third population showed a significant, but slight
increase in tolerance (LC50 at 24 hours: 29 g/g) while the fourth population had a
much higher LC50 (>200 g/g at 24 hours). In subsequent field trials carried out in
the apiary in which the latter population had been sampled, the total average
efficacy of two treatments with Perizin® (the commercial formulation of coumaphos)
in colonies without capped brood was 46% (28-88% ).